1. Field of the Invention
The invention in the field of biochemistry and medicine relates to novel proteins that are selectively expressed on the surface of dendritic cells and can be used as cell surface molecules or in soluble form in vaccine compositions to stimulate immune responses.
2. Description of the Background Art
The generation of a T lymphocyte response is a complex process involving cell-cell interactions and production of soluble mediators (cytokines or lymphokines). This response is regulated by several T-cell surface molecules acting as “receptors,” including the T-cell receptor (TCR) complex and other “accessory” surface molecules many of which are cell surface “differentiation antigens” that were first defined by monoclonal antibodies (“CD molecules”)
Optimal activation of all lymphocytes is believed to require two signals: an antigen specific or clonal signal, as well as a second, antigen non-specific signal (Janeway, C., Cold Spring Harbor Symp. Quant. Biol. 54:1-14 (1989)). If a lymphocyte encounters an antigen alone, without co-stimulation by so-called co-stimulatory molecules (such as B7 described below), it will respond with either clonal inactivation also called “anergy” (Schwartz, R. Science 248:1349 (1990)) or apoptosis (programmed cell death); if the co-stimulatory signal is provided it will respond with clonal expansion specific for the stimulating antigen. No significant amplification of an immune response against a given antigen occurs without co-stimulation (June et al. (Immunology Today 15:321-331, 1994); Chen et al. (Immunology Today 14:483-486); Townsend, S E and Allison, J P (1993) Science 259:368-370).
The quality and potency of an immune response depends in large part on the type of antigen presenting cells (APC) that process and present the antigen to T cells. The density of the peptide antigen/MHC ligand available for engagement of the TCR and the provision of soluble and/or membrane-bound co-stimulatory signals by APCs at the time of T cell engagement and activation is critical. It is for these reasons that immunotherapeutic strategies have begun to focus on providing (a) the target antigen to the appropriate APC types and (b) appropriate co-stimulatory molecules to enhance T cell activation.
APCs that provide the signals required for activation of T cells include monocytes/macrophages, B lymphocytes, and, most importantly dendritic cells (DCs). In the past, activated macrophages were believed to be the critical APCs that initiated T cell responses in vivo. This notion was based on their ability to phagocytose antigens effectively and process them for surface display and presentation. More recently, attention has shifted to DC as the major initiator in vivo of antigen-specific T cell responses. DCs have a distinct phenotype from activated macrophages and are classified into different subtypes capable of initiating distinct immune responses. A functional hallmark of DCs is their approximately 100-fold greater potency then macrophages to activate naïve T cells in vitro. To date, the explanation of this potency has been based on quantitative differences in molecules known to be important for antigen presentation. The present invention is based on the discovery of an important qualitative difference.
The first signal in antigen presentation is initiated by interaction of the TCR with antigen presented in the context of class II major histocompatibility complex (MHC) molecules on the APC (Allen, Immunol. Today 8:270 (1987)). Co-stimulatory signals come from other molecules, the best characterized of which is the B7 family (namely B7.1, B7.2, and possibly B7.3) which are also present on APCs
Two proteins expressed on the surface of T cells are the best-characterized ligands or counter-receptors for co-stimulatory molecules such as B7. CD28 is a homodimeric glycoprotein of the immunoglobulin (Ig) superfamily (Aruffo and Seed, Proc. Natl. Acad. Sci. 84:8573-8577 (1987)) found on most mature human T cells that functions in T cell activation. CD28, is constitutively expressed on resting T cells and increases after activation. After signaling through the T cell receptor, ligation of CD28 induces T cells to proliferate and secrete IL-2 (Linsley, P S, et al. (1991) J. Exp. Med. 173, 721-730; Gimmi, C D, et al. (1991) Proc. Natl. Acad. Sci. USA. 88, 6575-6579; Thompson, C. B., et al. (1989) Proc. Natl. Acad. Sci. USA. 86, 1333-1337; June, C. H., et al. (1990) Immunol. Today. 11, 211-6; Harding, F. A., et al. (1992) Nature. 356, 607-609.). CD28 mediates cell-cell contact (“intercellular adhesion”), a form of antigen-independent intercellular interaction that is essential for immune responses (Springer et al., Ann. Rev. Immunol. 5:223-252 (1987)).
CTLA4 is a T cell surface molecule highly homologous to CD28 but is not expressed on resting T cells and appears following T cell activation (Brunet, J. F., et al., (1987) Nature 328, 267-270). CTLA-4 was originally identified by differential screening of a murine cytolytic T cell cDNA library, Brunet et al. supra. The role of CTLA-4 as a second receptor for B7 is discussed in Linsley et al. (1991) J. Exp. Med. 174:561-569, which also noted that B7 has a higher affinity for CTLA4 than for CD28. Freeman et al. (1993) Science 262:907-909 discussed CTLA-4 in B7 deficient mice. Ligands for CTLA-4 are described in Lenschow et al. (1993) Proc. Nat'l. Acad. Sci. 90:11054-11058.
Th cells secrete growth and differentiation-inducing cytokines such as IL-2, IL-4 and IL-6 possibly in a focused manner in the area of Th-B cell contact which serves to ensure activation of only B cells presenting antigen to Th cells and avoid activation of bystander B cells.
CD28 and CTLA-4 interact with a co-stimulatory molecule generally known as B7. B7 was originally described as a B cell activation antigen because it was found on B cells and was termed B7/BB-1 (Linsley et al, Proc. Natl. Acad. Sci. USA 87:5031-5035 (1990). Hereafter, this molecule will be referred to as B7, B7-1 or B7.1). B7 and more recently described B7 homologues are also members of the Ig superfamily. In contrast to CD28 and CTLA-4, B7 comprises two extracellular Ig domains, an N-terminal variable (V)-like domain followed by a constant (C)-like domain.
B7 family members are generally expressed on APCs and, as noted, are of critical importance to the activation of naive T cells. These family members include B7-1 (=B7, also designated CD80) and B7-2 (also designated CD86). References describing B7-1 include Schwartz, R. H. Cell 71:1065-1068, 1992; Chen, L. et al. Cell 71:1093-1102, 1992; Freeman, G. J. et al. J. Immunol 143:2714-2722, 1989; and Freeman, G. J. et al. J. Exp. Med. 174:625-631, 1991)). References describing B7-2 include (Freeman, G. J. et al. Science 262:909-911 813-960, 1993). To date, both murine B7-1 and B7-2 and human B7-1 and B7-2 have been described (Freeman et al., 1989, supra; 1991, supra; and 1993, supra). Activated human B lymphocytes express CTLA4/CD28 binding counter-receptors B7-2 and B7-3, both of which can deliver costimulatory signals to T cells via either CD28 or CTLA4.
B7-2 is expressed by B cells at about 24 hours following stimulation with either anti-Ig or anti-MHC class II mAbs. B7-2 induces detectable IL-2 secretion and T cell proliferation. At about 48 to 72 hours post activation, B cells express both B7-1 and a third CTLA4 counter-receptor identified by a mAb BB-1 (Yokochi, T, et al. (1982) J. Immunol. 128, 823-827), termed B7-3. B7-3 is also expressed on B7-negative activated B cells and can costimulate T cell proliferation without detectable IL-2 production, indicating that the B7-1 and B7-3 molecules are distinct. B7-3 is expressed on a wide variety of cells including activated B cells, activated monocytes, dendritic cells, Langerhans cells and keratinocytes. At 72 hours post B cell activation, the expression of B7-1 and B7-3 begins to decline. The presence of these CTLA4/CD28 binding counter-receptors on the surface of activated B lymphocytes indicates that T cell costimulation is regulated, in part, by the temporal expression of these molecules following B cell activation.
The importance of the B7:CD28/CTLA4 costimulatory pathway(s) has been demonstrated in vitro and in vivo. A direct relationship exists between increased T cell activity and increased B7 expression (Razi-Wolf et al., Proc. Natl. Acad. Sci. USA, 89:4210-4214 (1992)). T cells are rendered anergic when they, encounter peptides antigens on cells lacking a costimulatory ligand that binds CD28 Blockade of this costimulatory pathway results in the development of antigen specific tolerance in murine and humans systems (Harding et al., supra; Lenschow, D. J. et al. (1992) Science. 257, 789-792; Turka, L A et al. (1992) Proc. Natl. Acad. Sci. USA. 89, 11102-11105; Gimmi, C D et al. (1993) Proc. Natl. Acad. Sci USA 90, 6586-6590; Boussiotis, V. et al. (1993) J. Exp. Med. 178, 1753-1763). Conversely, expression of B7 by B7-negative murine tumor cells induces T-cell mediated specific immunity accompanied by tumor rejection and long lasting protection to tumor challenge (Chen, L, et al. (1992) Cell 71:1093-1102; Townsend et al., supra; Baskar, S, et al. (1993) Proc. Natl. Acad. Sci. 90, 5687-5690.). Therefore, manipulation of the B7:CD28/CTLA4 pathway offers great potential to stimulate or suppress immune responses in humans.
Interactions between CD28 and B7 have been characterized using genetic fusions of the extracellular portions of B7 or CD28 with Ig Cγ1 chains (Linsley et al, J. Exp. Med. 173:721-730 (1991)). When B7Ig fusion proteins are immobilized, or when B7 is expressed on the surface of a cell, such as a transfected CHO cell, they costimulate T cell proliferation. T cell stimulation with B7+ CHO cells also specifically stimulates increased levels of transcripts for IL-2.
U.S. Pat. No. 5,521,288 describes a method for regulating immune responses by contacting CD28 positive T cells with fragments encoded by parts of DNA encoding B7, primarily corresponding to the extracellular domain (ECD) of B7. Immune responses were also regulated by derivatives of B7 that were are fusion protein constructs including at least a portion of B7 ECD and another protein, such as the human IgCγ1 domain that altered the solubility, binding affinity and/or valency of B7. For example DNA encoding amino acid residues from positions 1-215 of the B7 ECD was joined to DNA encoding amino acid residues of the sequences corresponding to the hinge, CH2 and CH3 regions of human IgCγ 1 to form a DNA fusion product which encoded a B7Ig fusion protein. Also disclosed was a method for treating an immune system disease mediated by T cells by administering B7 or B7Ig fusion protein to react with T cells by binding the CD28 receptor. T cell proliferation in graft versus host disease was inhibited by reacting CD28+ T cells with B7 antigen or B7Ig fusion protein in combination with an immunosuppressant.
U.S. Pat. No. 5,861,310 discloses tumor cells modified to express one or more T cell costimulatory molecules, including B7-2 and B7-3. One embodiment includes further expression of B7. Modification was by transfection with nucleic acid encoding the B7-2, B7-3 or B7 proteins. Tumor cells could also be genetically modified in vivo. Such modified tumor cells said to be useful for treating a patient with a tumor, to prevent or inhibit metastatic spread or inhibit recurrence of the tumor. This document disclosed a method for specifically inducing a CD4+ T cell response against a tumor.
U.S. Pat. No. 5,942,607 discloses isolated nucleic acids encoding novel CTLA4/CD28 ligands which costimulate T cell activation. In one embodiment, the isolated nucleic acid encoded B7-2. Also disclosed was a nucleic acid comprising at least a portion of the disclosed full length B7-2 sequence. According to this document, the nucleic acid sequences could be integrated into various expression vectors which could direct the synthesis of the corresponding proteins or peptides in a variety of host cells including mammalian and insect cells. Also disclosed were host cells transformed to produce proteins or peptides encoded by these nucleic acid sequences and isolated proteins and peptides which comprise at least a portion of the B7-2 sequence.
Dong H et al., Nat Med 1999 5:1365-1399, described a third member of the B7 family, designated B7-H1 that does not bind CD28, CTLA4 or ICOS (inducible co-stimulator). Ligation of B7-H1 co-stimulated T-cell responses to polyclonal stimuli and alloantigens, and preferentially stimulated the production of interleukin-10. IL-2, produced in small amounts, was required for the effect of B7-H1 co-stimulation. This study defined a previously unknown co-stimulatory molecule that may be involved in the negative regulation of cell-mediated immune responses. The same laboratory (Wang S et al., Blood. 2000; 96:2808-2813) described a new human B7-like gene designated B7-H2, the expression of which was detected on the surface of monocyte-derived immature DCs. Soluble B7-H2 and an Ig fusion protein, B7-H2Ig, bound to activated, but not resting, T cells. This binding was inhibited by a soluble form of ICOS (ICOSIg) but not by CTLA4Ig. ICOSIg stained CHO cells transfected with the B7-H2 gene. Using suboptimal cross-linking of CD3 as a stimulus, costimulation of T-cell proliferation by B7-H2Ig was found to be dose-dependent and correlated with secretion of IL-2, whereas optimal CD3 ligation preferentially stimulated IL-10 production. The authors concluded that B7-H2 is a putative ligand for the ICOS T-cell molecule.
Swallow MM et al., Immunity, 1999, 11:423-432 reported cloning of a novel gene, b7h, a is a close homolog of B7 molecules that are expressed on APCs. B7h costimulated proliferation of purified T cells by acting on a receptor distinct from CD28 or CTLA-4. Surprisingly, although B7h was expressed in unstimulated B cells, its expression was induced in nonlymphoid cells (3T3 cells; embryonic fibroblasts) treated with TNFα and was upregulated in nonlymphoid tissue of mice treated with LPS, a potent activator of TNFα. These studies defined a novel costimulatory ligand of T cells and suggested that induction of B7h by TNFα may directly augment recognition of self during inflammation
Yoshinaga S K et al., Nature, 1999, 402:827-832, described a new murine costimulatory receptor-ligand pair. The receptor, related to CD28, was the murine homologue of the human protein ICOS, and was expressed on activated T cells and resting memory T cells. The ligand, which was homologous to B7 molecules was designated B7-related protein-1 (B7RP-1). B7RP-1 is a type 1 transmembrane protein with 20% and 19% amino acid identity to murine B7.1 (CD80) and B7.2 (CD86), respectively. This homology is significant as B7.1 and B7.2 share only 27% amino acid identity (Freeman, G J et al., J. Exp. Med. 178:2185-2192 (1993)). This homology includes the cysteines that are important for Ig loop formation at conserved locations (residues 62, 138, 185 and 242 from the initiating methionine). The overall length and relative position of the transmembrane domain of B7RP-1 are similar to those of the B7 molecules (Greenfield, E A et al., Crit. Rev. Immunol. 18:389-418 (1998)). B7RP-1 was shown to be expressed on B cells and macrophages. ICOS and B7RP-1 did not interact with proteins in the CD28-B7 pathway, and B7RP-1 co-stimulated T cells independently of CD28. Transgenic mice expressing a fusion protein between B7RP-1 and the Fc portion of Ig (“B7-RP1-Fc”) had lymphoid hyperplasia in spleen, lymph nodes and Peyer's patches. Co stimulatory activity of B7RP-1 in vivo was found by demonstrating enhanced delayed-type hypersensitivity in antigen-presensitized mice treated with B7RP-1-Fc at the time of antigen challenge. The authors concluded that ICOS and B7RP-1 define a distinct new receptor-ligand pair that is structurally related to CD28-B7 and is involved in the adaptive immune response.
Yoshinaga S K et al., Int Immunol, 2000 October, 12:1439-1447, reported co-stimulation of human T cells through the human B7RP-1 and ICOS interaction. This ligand-receptor pair interacted with a KD of approximately 33 nM and an off-rate having a t(1/2) of >10 min. TNFα differentially regulated expression of human B7RP-1 on B cells, monocytes and DC. TNFα enhanced B7RP-1 expression on B cells and monocytes, but inhibits expression on DC. A human B7RP-1-Fc protein, or cells that expressed membrane-bound B7RP-1, co-stimulated T cell proliferation in vitro. Specific cytokines, such as IFNγ and IL-10, were induced by B7RP-1 co-stimulation. Although IL-2 levels were not significantly increased, B7RP-1-induced co-stimulation was dependent on IL-2. These studies defined the human ortholog to murine B7RP-1 and characterized its interaction with human ICOS.
PD-1 is an immuno-inhibitory receptor expressed by activated T, B and myeloid cells. Mice deficient in PD-1 showed multiple forms of autoimmunity due to the loss of peripheral tolerance. Freeman, G J et al., J. Exp. Med. 192:1027-1034 (2000) reported that the ligand of PD-1 (PD-L1) is a member of the B7 gene family. Engagement of PD-1 by PD-L1 resulted in inhibition of TCR-mediated lymphocyte activation (proliferation, cytokine secretion). In addition, PD-1 signaling inhibited suboptimal levels of CD28-mediated costimulation. PD-L1 is expressed by APCs (human monocytes stimulated with IFNγ, activated human DCs). In addition, PD-L1 was shown to be is expressed in heart and lung. The authors speculated that relative magnitude of inhibitory PD-L1 signals and costimulatory B7-1/B7-2 signals on APCs may determine the extent of T cell activation and the threshold between tolerance and autoimmunity. The presence of PD-L1 on nonlymphoid tissues may contribute to the magnitude of immune responses at sites of inflammation.
Citation of the above documents is not intended as an admission that any of the foregoing is pertinent prior art. All statements as to the date or representation as to the contents of these documents is based on the information available to the applicant and does not constitute any admission as to the correctness of the dates or contents of these documents.